Most conventional semiconductor power devices are almost exclusively formed using silicon (Si). Due to the relative maturity of the use of this semiconductor, the performance of conventional power devices to carry high currents and block high voltages is closely approaching the theoretical limit for Si. For example, power MOSFETs made using Si have undergone many improvements over the past two decades and allowing them to block 30 to 1,200 volts while providing relatively low on-state resistance values.
However, there are many applications for power devices that require the ability to carry high currents and block voltages in the range of 600 V to 15 kV (and greater). These applications include motor control, power supply, lighting ballast, power transmission and generation, and utility power conversion. Unfortunately, the overall performance of power devices made using Si is poor for this voltage range, and the lack of such power devices represents the primary limitation in realizing circuits for these applications. In fact, if high voltage devices that support such high currents and operate at frequencies of one to 100 kHz were available, they would revolutionize power utility applications and result in power savings of as much as $20 billion in the United States.
One recent development in semiconductor power devices is the use of Intelligent Power Modules (IPMs). IPMs use low voltage CMOS circuitry that may be integrated with power devices. Other examples of intelligent power devices include discrete integrated power devices that detect unacceptable current, voltage, and temperature conditions. However, the relatively low blocking voltage of semiconductor power devices made using Si limits the application of these devices in majority carrier devices (e.g., devices that rely on resistive current transport) to 1200 V or less.
For higher power devices (e.g., those blocking voltages greater than 1200 V), bipolar devices, such as, insulated gate bipolar transistors (IGBTs) and Thyristors have been used. While these devices offer acceptable on-state performance, they suffer from relatively slow switching speeds and poor performance at high temperatures.
Other power devices that have been researched also suffer from various deficiencies. For example, Bipolar Junction Transistors (BJTs) use a current control gate rather than a preferable voltage control gate. Many vertical junction field effect transistors (JFETs) operate in a “normally-on” mode during their on-state condition; however, JFETs with the preferable “normally-off” mode have poor on-state resistances. Finally, thyristors have high on-state voltage drops (because of their inherent junction drop) and slow switching speeds.
A variety of power devices using silicon carbide (SiC) have been researched and implemented in an attempt to provide devices that block high voltages and carry high currents. One switching power device is the vertical power MOSFET. However, vertical power MOSFETs made using SiC suffer from poor performance and poor reliability because of low inversion layer channel mobility. In addition, the tunneling current between SiC and the gate dielectric limits the reliability of these power devices made using SiC during their long term operation.
Another area in which semiconductor devices are being used is the microwave transistor. Microwave transistors are expected to work at very high frequencies, for example, at 800 MHz to 10 GHz (and greater). Most conventional microwave devices are formed using gallium arsenide (GaAs) and Si structures. For example, GaAs structures include the high electron mobility transistor (HEMT), the Heterojuncntion Bipolar transistor (HBT), and the pseudomorphic HEMT. These transistors are the most commonly used lower power (e.g., fractional to 10 W) transistors. Their applications include cell phone handsets and low power networking devices.
Higher power (e.g., especially in the L-Band (400 MHz to 1 GHz), S-Band (1–3 GHz) and X-band (7–10 GHz)) transistor applications also exist including military radars and cellular phone base-station applications. These transistors may be combined with passive components to form integrated circuits that provide higher usable power levels and match external circuit components. Such integrated circuits are called microwave and millimeter wave integrated circuits (MMICs).
One example of a high power microwave device formed using SiC is the lateral power Metal-Semiconductor FET (or MESFET). A MESFET is operated by controlling the source to drain current using a Schottky gate located between these terminals (known as a T-gate). Due to the extremely high current densities in the localized region inside the MESFET, the junction temperature at the Schottky contact may significantly exceed the ambient temperature and result in a high leakage current at the gate and premature breakdown. As a result, the source terminal of the microwave MESFET must be firmly held at ground potential to achieve stable performance.
To hold the source terminal of a microwave MESFET at ground, via holes inside the source terminal must be placed through the wafer to the backside metal contacts. As a result, the yield of chips made using ultra-fine lithography is reduced (and therefore the cost of producing such chips is increased). Because the T-gate significantly increases the cost of making these chips, the use of MESFETs for many high power density microwave applications may ultimately be limited.
As an alternative to MESFETs, microwave JFETs with p-type gate regions located between the source and drain contacts have been used in an attempt to provide better robustness. However, JFETs also suffer from poor high frequency performance in microwave applications because of the challenge in making small gate widths that minimize gate to source and gate to drain capacitances.